1. Field of the Invention
The present invention relates to an amplifier that includes an AGC circuit and an electronic variable resistor circuit as built-in elements, and in particular to an amplifier having a configuration in which a variable gain amplifier is shared for composing the AGC circuit and the electronic variable resistor.
2. Description of Related Art
The miniaturization of electronic components and reduction of the number of components used in portable devices such as digital still cameras and digital video cameras is considered to be of importance in the miniaturization and reduction of weight for such devices.
In an AGC (Automatic Gain Control) circuit used for improving the distortion characteristics of a handled signal, by converting a detection signal obtained by detecting a level of an output signal to a digital signal, it becomes possible to eliminate an external component. On the other hand, there are also many cases where an EVR (Electronic Variable Resistor) function for gain switching is provided in products that handle audio signals.
FIG. 6 illustrates an audio amplifier that has an AGC circuit and an electronic variable resistor circuit, as disclosed in JP H7-34534A. A signal inputted from an input terminal 1 is amplified by a variable gain amplifier 31 and outputted from an output terminal 2. Signals outputted from a gain control circuit 32 and a detection circuit 33 are supplied to the variable gain amplifier 31. A control signal is supplied to the gain control circuit 32 from a control signal input terminal 30 via a switch 34, and the electronic variable resistor circuit is configured thereby. The signal outputted from the variable gain amplifier 31 is supplied to the detection circuit 33 via a switch 35 and a rectifier circuit 36. Furthermore, a smoothing capacitor 37 is connected between the point at which the rectifier circuit 36 and the detection circuit 33 are connected and a ground, and the AGC circuit is configured thereby.
The AGC circuit and the electronic variable resistor circuit are both circuits that realize a function for controlling the gain of the amplifier. When the level of the input signal fluctuates, the AGC circuit detects the output signal level through the detection circuit and adjusts the gain of the variable gain amplifier based on the detected signal so that the output signal achieves a desired level. On the other hand, the electronic variable resistor circuit does not need to control the gain of the variable gain amplifier as long as the set gain is not altered.
For this reason, when an AGC circuit and an electronic variable resistor circuit are included within the same semiconductor integrated circuit, oftentimes they conventionally have been composed of individual elements, which has led to an increase in the scale of the circuit. Moreover, control of the electronic variable resistor circuit has been performed through ON/OFF selection of plural variable gain amplifiers, which, when used in audio applications, has been a cause of “popping” sounds due to DC voltage fluctuations in the output voltage.
Furthermore, in the example of conventional art illustrated in FIG. 6, the amplifier is configured so that the AGC circuit and the electronic variable resistor circuit share the variable gain amplifier, and the amplifier utilizes the smoothing capacitor 37, which is normally implemented as an external component, to detect the signal outputted from the AGC circuit; causing an obstacle in the reduction of the number of components in the system as a whole.